1. Field of the Invention
The present invention relates to a radio frequency receiver of twin-tuner type provided with two systems of receivers.
2. Description of the Prior Art
There has been an increasing demand for a radio receiver to output demodulated signal of higher reception quality, and many kinds of proposals have been made so far concerning the above.
For example, a receiver having such function is being proposed which can detect stations having higher electric field intensity in a specified area automatically, write some of those stations into a preset assigned area in a memory in the order of the height of electric field, and call the stored stations in the memory by the preset button operation.
Such a function is called BSM (Best Stations Memory), which function includes sweeping of received frequencies, for example, from low end to upper end of the reception band, writing of frequencies having higher electric field intensity than that of the specified level into the memory, and presetting of the frequencies in the order of the intensity level of electric field in accordance with the preset button etc.
One example of the radio receiver provided with the above-mentioned functions is shown in FIG. 6.
In FIG. 6, numerical 1 designates a radio frequency (RF) receiving antenna, and the RF signal received through this antenna 1 is amplified by an RF amplifier 2 and then applied to a mixer 3. Local signal outputted from a local signal oscillator 4 constituted by, for example, a PLL circuit is fed to the mixer 3 and the received signal is here converted to an intermediate frequency signal.
Intermediate frequency signal thus obtained by the mixer 3 is selectively amplified by an intermediate frequency amplifier 5 and demodulated by a detector 6 of the next stage. The demodulated signal output by the detector 6 is further stereo-demodulated by a multiplexing circuit 7 and passes through a muting circuit 8 and is applied to power amplifiers 9L, 9R for amplifying left and right sound signals.
Then, the demodulated sound signals amplified by the power amplifiers 9L, 9R are reproduced by loudspeakers 10L, 10R.
On the other hand, the output signal produced by detecting the level of the intermediate frequency signal, in other words, the output signal corresponding to the reception electric field intensity is applied from the intermediate frequency amplifier 5 to a central processing unit 11 comprising a microprocessor.
A keyboard 12 by which the desired frequencies etc. to be received are inputted, a display 13 indicating the frequencies etc. to be received and the local signal oscillator 4 constituted by the above-mentioned PLL circuit are all connected to the central processing unit 11.
In the constitution as mentioned above, when the BSM function as above is operated during the RF reception, the mute control signal from the central processing unit 11 is applied to the muting circuit 8 so as to prevent the frequency being presently received and reproduced from being demodulated. Immediately after the above operation, the central processing unit 11 sends a command to the local signal oscillator 4 for sweeping the frequencies to be received from low end to high end of the reception band, and the receiver begins to scan the tuning operation of the frequencies sequentially.
At this time, the reception level outputted from the intermediate frequency amplifier 5 is applied to the central processing unit 11, and when the outputted reception level is equal to or higher than a predetermined level, the central processing unit 11 stores the level value and the corresponding received frequency at that time.
When the tuning scanning from low end to high end of the reception band is completed, the central processing unit 11 presets the received frequencies corresponding to preset buttons according to their reception levels. The central processing unit 11 then outputs the control signal to the local signal oscillator 4 in order to tune the frequency of the maximum frequency level, and then sends a signal for clearing the muting function to the muting circuit 8, so that the receiver results in receiving and reproducing the highest reception level of the frequency in that area.
In case the above-mentioned BSM operation is intended to be carried out, for example, referring to FM band in Japan, a frequency band 76-90 MHz will be swept by step of every 0.1 MHz. Assuming the time required for sweeping at each step is 50 msec, the time necessary to sweep from low end to high end of the FM band becomes 7 sec +, during which time the reception is thus muted, so that it is regarded as a waiting time.
Apart from the above-mentioned BSM function which utilizes means for sweeping from the low end to high end of one of FM band, another BSM function has also been proposed heretofore, wherein the detecting level of the frequency is set to high level in the first sweep, that is, the level of the frequency reception is set to the "LOCAL" position so as to extract the broadcasting station received thereby, and the detecting level is then lowered in the second sweep, that is, the level of the frequency reception is, this time, set to the "DX" position so as to extract the broadcasting station received thereby. By this operation, broadcasting stations are received in the order of the height of its reception sensitivity.
However, the BSM function as above requires twice as long as the above-mentioned waiting time.
As described above, the radio receiver according to the prior art requires a considerable waiting time, for example, during the operation of BSM function, and consequently a user can not obtain any information from the radio receiver during such waiting time.